During combustion of coal in a fluidized bed of a particulate sulphur-absorbing material, for example lime or dolomite, a large quantity of ashes from the fuel and fine-grained absorbent residues accompany the flue gases. This dust is separated from the flue gases in a cleaning plant, usually consisting of cyclones, before the gases are utilized for operation of a gas turbine. In the following, the separated dust will be referred to as cyclone ash. The combustion is performed at a pressure considerably exceeding the atmospheric pressure. The pressure may be about 20 bar, is usually between 12 and 16 bar at full power, but is lower at partial power. The combustion of the fuel is performed in the bed at a temperature of about of 850.degree. C. Combustion gases and accompanying dust have the same temperature as the bed. Also the separated dust, the cyclone ash, has this high temperature. Therefore, the handling of ashes entails considerable problems.
To be able to handle ashes, the following must be done:
1. The cyclone ash must be cooled to &lt;100.degree. C., preferably to &lt;70.degree. C. Cooling to this low temperature is necessary to permit the storage of the ash in ash silos of an inexpensive type, such as concrete silos, and to permit transportation of the ash by conventional bulk transport devices.
2. The pressure must be reduced from 3-16 bar to atmospheric pressure.
3. The temperature must be reduced to permit transportation of separated dust by simple transport devices to ash silos which must often be located at a considerable distance from the gas cleaning plant. Distances of 100-300 m are common.
4. Flue gases must be separated from the cyclone ash before the ash is cooled to a temperature which is below the dew point of sulphuric acid. The dew point is dependent on the pressure level, the moisture content, and the content of sulphur dioxide in the flue gases, which are used for pneumatic transport of the cyclone ash, and is generally between 100 and 180.degree. C. Otherwise, sulphuric acid condenses on cooling surfaces at temperatures below the dew point and ash particles form a growing coating on the cooling surfaces until the external temperature of the coating becomes equal to or exceeds the dew point in question.
In known PFBC power plants, the cyclone ash is cooled from approximately 700.degree. C. in two stages. In the first stage, the compressed combustion air is usully used as coolant, and in this first cooling stage the cooler may be a pressure-reducing ash discharge device which is located together with the combustor in a pressure vessel. The air temperature is, after the compression, 250-300.degree. C. and makes possible cooling to 300-400.degree. C. An ash discharge device of the above-mentioned kind designed as a cooler is described in European Patent No. 0 108 505.
In a second cooling stage, the cyclone ash may be cooled with water and the heat contents be utilized for preheating of, for example, feed water or distance heating water. The fine-grained state and poor thermal conductivity of the cyclone ash render the cooling difficult. To obtain good contact between ash and cooling surfaces, the cyclone ash is suitably fluidized in the cooler. Discharge of heat with the fluidization air entails an undesirable heat loss.
Swedish patent application 8802526-7 shows a cooler designed as a water-cooled transport screw. U.S. Pat. No. 4,492,184 shows a cooler designed as an inclined bed vessel where cyclone ash forms the bed.